Millions of Americans spend billions of dollars on vitamins to boost their health. Research has focused on cancer and heart disease in particular, because of evidence that diets full of vitamin-rich foods may protect against those illnesses. But that evidence doesn't necessarily mean pills are a good substitute.

The study's lead author, researcher Marian Neuhouser of the Fred Hutchinson Cancer Research Center in Seattle, offered this advice: "Get nutrients from food. Whole foods are better than dietary supplements," Neuhouser said.

For one thing, the data are observational, not the most rigorous kind of scientific research. And also, it's not clear if taking vitamins might help prevent cancers that take many years to develop, said Manson, chief of preventive medicine at Harvard's Brigham & Women's Hospital.

She said multivitamins may still be useful "as a form of insurance" for people with poor eating habits.

Alice Lichtenstein, a Tufts University nutrition professor who was not involved in the research, said the study is important because it involved so many women. "All the evidence keeps pointing in the same direction," Lichtenstein said.